Trench formation using horn shaped spacer

ABSTRACT

A method includes forming a mandrel layer over a target layer, and etching the mandrel layer to form mandrels. The mandrels have top widths greater than respective bottom widths, and the mandrels define a first opening in the mandrel layer. The first opening has an I-shape and includes two parallel portions and a connecting portion interconnecting the two parallel portions. Spacers are formed on sidewalls of the first opening. The spacers fill the connecting portion, wherein a center portion of each of the two parallel portions is unfilled by the spacers. Portions of the first opening that are unfilled by the spacers are extended into the target layer.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No.14/840,162, entitled “Trench Formation using Horn Shaped Spacer,” filedon Aug. 31, 2015, which application is a continuation of U.S. patentapplication Ser. No. 14/097,617, entitled “Trench Formation using HornShaped Spacer,” filed on Dec. 5, 2013, now U.S. Pat. No. 9,136,162,issued Sep. 15, 2015, which applications are incorporated herein byreference.

BACKGROUND

Double patterning is a technology developed for lithography to enhancethe feature density. Typically, for forming features of integratedcircuits on wafers, the lithography technology is used, which involvesapplying a photo resist, and defining features on the photo resist. Thefeatures in the patterned photo resist are first defined in alithography mask, and are implemented either by the transparent portionsor by the opaque portions in the lithography mask. The features in thepatterned photo resist are then transferred to the manufacturedfeatures.

With the increasing down-scaling of integrated circuits, the opticalproximity effect posts an increasingly greater problem. When twoseparate features are too close to each other, the optical proximityeffect may cause the features to short to each other. To solve such aproblem, double patterning technology is introduced. In the doublepatterning technology, the closely located features are separated to twophotolithography masks of a same double-patterning mask set, with bothmasks used to expose the same photo resist, or used to pattern the samehard mask. In each of the masks, the distances between features areincreased over the distances between features in the otherwise a singlemask, and hence the optical proximity effect is reduced, orsubstantially eliminated in the double patterning masks.

The double patterning, however, also suffers from drawbacks. Forexample, when two features have their lengthwise directions aligned to asame straight line, and the line ends of the features face each other,it is difficult to control the uniformity of the line end space due tothe proximity effect and overlay variation. The line widths of thefeatures are also difficult to control, especially when there are otherfeatures close to these two features.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments, and the advantagesthereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, in which:

FIGS. 1A through 11 are top views and cross-sectional views ofintermediate stages in the formation of features in a target layer inaccordance with some exemplary embodiments; and

FIGS. 12A through 12C illustrate a top view and cross-sectional views ofthe features formed in the target layer.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the disclosure are discussedin detail below. It should be appreciated, however, that the embodimentsprovide many applicable concepts that can be embodied in a wide varietyof specific contexts. The specific embodiments discussed areillustrative, and do not limit the scope of the disclosure.

Features with fine line spacing and the methods of forming the same areprovided in accordance with various exemplary embodiments. Theintermediate stages of forming the features are illustrated inaccordance with some exemplary embodiments. The variations of theembodiments are discussed. Throughout the various views and illustrativeembodiments, like reference numbers are used to designate like elements.

FIGS. 1A through 11 illustrate top views and cross-sectional views ofintermediate stages in the formation of features in a target layer inaccordance with some exemplary embodiments. Some of the top-view figuresfurther include line C-C, wherein the cross-sectional view of the samestructure is obtained from the horizontal plane containing line C-C inthe top view.

FIGS. 1A and 1B illustrate a top view and a cross-sectional view,respectively, of wafer 100, which includes substrate 120 and theoverlying layers. Substrate 120 may be formed of a semiconductormaterial such as silicon, silicon germanium, or the like. In someembodiments, substrate 120 is a crystalline semiconductor substrate suchas a crystalline silicon substrate, a crystalline silicon carbonsubstrate, a crystalline silicon germanium substrate, a III-V compoundsemiconductor substrate, or the like. Active devices 122, which mayinclude transistors therein, are formed at a top surface of substrate120.

Dielectric layer 124 is formed over substrate 120. In some embodiments,dielectric layer 124 is an Inter-Metal Dielectric (IMD) or anInter-Layer Dielectric (ILD), which may be formed of a dielectricmaterial having a dielectric constant (k value) lower than 3.8, lowerthan about 3.0, or lower than about 2.5, for example. In someembodiments, conductive features 126, which may be metallic featuressuch as copper lines or tungsten contact plugs, are formed in dielectriclayer 124. Etch stop layer 26 is formed over dielectric layer 124. Etchstop layer 26 may comprise a dielectric material such as siliconcarbide, silicon nitride, or the like.

In some embodiments, as shown in FIG. 1B, oxide layer 27 is formed overetch stop layer 26. Oxide layer 27 acts as a glue layer to improve theadhesion between etch stop layer 26 and the overlying dielectric layer28. In some embodiments, oxide layer 27 comprises atetraethylorthosilicate (TEOS) oxide.

Dielectric layer 28 is further formed over oxide layer 27. Dielectriclayer 28 may be an IMD layer, which is formed of a dielectric materialhaving a dielectric constant (k value) lower than 3.8, lower than about3.0, or lower than about 2.5, for example. Dielectric layer 28 maycomprise carbon, and may have pores therein. In alternative embodiments,dielectric layer 28 is a non-low-k dielectric layer having a k valuehigher than 3.8.

In alternative embodiments, layer 28 is a semiconductor substrate. Inthese embodiments, there may not be additional layers underlying layer28. Hence, the illustrated layers 120, 124, 26, and 27 as shown in FIG.1B may not be formed. Throughout the description, layer 28 is alsoreferred to as a target layer, in which a plurality of patterns is to beformed therein in accordance with the embodiments of the presentdisclosure.

Over low-k dielectric layer 28 resides dielectric hard mask 30, whichmay be formed of silicon oxide (such as TEOS oxide), Nitrogen-FreeAnti-Reflective Coating (NFARC, which is an oxide), silicon carbide,silicon oxynitride, or the like. The formation methods include PlasmaEnhance Chemical Vapor Deposition (PECVD), High-Density Plasma (HDP)deposition, or the like.

Mandrel layer 36 is formed over dielectric hard mask 30. In someembodiments, mandrel layer 36 is in contact with dielectric hard mask30, with no additional layer formed between mandrel layer 36 anddielectric hard mask 30. In some embodiments, mandrel layer 36 is formedof amorphous silicon or another material that has a high etchingselectivity with the underlying dielectric hard mask 30.

FIGS. 1A and 1B illustrate a first photolithography process. Overmandrel layer 36 (FIG. 1B) is formed a tri-layer comprising under layer(sometimes referred to as a bottom layer) 38, middle layer 40 over underlayer 38, and upper layer 42 over middle layer 40. In some embodiments,under layer 38 and upper layer 42 are formed of photo resists, whichcomprise organic materials. Middle layer 40 may comprise an inorganicmaterial, which may be a nitride (such as silicon nitride), anoxynitride (such as silicon oxynitride), an oxide (such as siliconoxide), or the like. Middle layer 40 has a high etching selectivity withrelative to upper layer 42 and under layer 38, and hence upper layer 42is used as an etching mask for the patterning of middle layer 40, andmiddle layer 40 is used as an etching mask for the patterning of underlayer 38. After the application of upper layer 42, upper layer 42 ispatterned. Upper layer 42 is patterned to form opening 44. The formationof under layer 38, middle layer 40, and upper layer 42 and the exposureand the development of upper layer 42 are referred to as a firstphotolithography (photo) process.

Referring to FIGS. 2A and 2B, which include a top view and across-sectional view, respectively, a first etching process is performedto transfer the pattern in upper layer 42 into mandrel layer 36. Therespective step is also referred to as a cut etch step. During theetching step, upper layer 42, middle layer 40, and under layer 38 may beconsumed. If any residue part of upper layer 42, middle layer 40, andunder layer 38 is left after the etching of mandrel layer 36, theresidue part is removed. FIGS. 2A and 2B illustrate the resultingstructure. The etching is anisotropic, so that the opening 44 in mandrellayer 36 has the same or similar size as the respective opening 44 inupper layer 42 (FIG. 1B).

FIGS. 3A, 3B, 4A, and 4B illustrate a second photolithography and asecond etching process performed on mandrel layer 36. Referring to FIGS.3A and 3B, which include a top view and a cross-sectional view,respectively, bottom layer 48, middle layer 50, and upper layer 52 areformed. The material of bottom layer 48, middle layer 50, and upperlayer 52 may be selected from the same candidate materials of bottomlayer 38, middle layer 40, and upper layer 42 (FIG. 1B), respectively.Upper layer 52 is patterned to form openings 54 therein.

Next, as shown in FIGS. 4A and 4B, which include a top view and across-sectional view, respectively, the second etching process isperformed to extend opening 54 into mandrel layer 36. As a result,mandrel layer 36 includes both openings 44 and 54. The remainingportions of mandrel layer 36 are referred to as mandrels 56 hereinafter.As shown in the top view in FIG. 4A, two of openings 54 areinterconnected by opening 44 to form an I-shaped opening. It is realizedthat openings 54 and opening 44 have portions overlapping each other. Inthe following description, opening 44 refers to the portioninterconnecting portion connecting openings 54, while the overlappedportions are considered as parts of openings 54.

In accordance with the embodiments of the present disclosure, in theetching of mandrel layer 36, process conditions for etching mandrellayer 36 are adjusted, so that remaining mandrels 56 have upper widthsW1 greater than the respective bottom widths W2. In some embodiments,mandrels 56 have inversed trapezoid shapes, with edges 56′ substantiallystraight in the side view. Edges 56′ may also be curved in alternativeembodiments. Tilt angles α of edges 56′ is smaller than 90 degrees. Insome embodiments, tilt angle α is between about 60 degrees and about 80degrees, or may be between about 60 degrees and about 85 degrees.

In some exemplary embodiments, the etching of mandrel layer 36 isperformed in a process chamber (not shown), which includes process gasessuch as CF₄, HBr, Cl₂, O₂, or combinations thereof. The flow rate of theprocess gases may be in the range between about 3 sccm and about 500sccm. The pressure of the process gases may be in the range tween about5 mtorr and about 50 mtorr. The etching may be performed with ahigh-frequency power applied for isotropic etching and a low-frequencypower applied for anisotropic etching at the same time. For example, thehigh-frequency power may have the frequency of 13.6 MHz, and thelow-frequency power may have the frequency of 2 MHz. The bias voltage islower than about 200 volts. During the etching, the temperature of wafer100 may be between about 15° C. and about 50° C. The process conditionsare adjusted so that while the etching includes an anisotropiccomponent, an isotropic effect is generated and increased to generatethe desirable profile for mandrel 56. For example, increasing thepressure of the process gases, increasing the amount of the etchinggases (such as O₂, HBr, and Cl₂) in the process gases, increasing thehigh-frequency power, and/or lowering the low-frequency power have theeffect of increasing the isotropic effect of the etching, and hencegenerating the desirable profile for mandrels 56. The optimal processconditions are related to various factors, and may be found throughexperiments.

Referring to FIGS. 5A and 5B, which include a top view and across-sectional view, respectively, spacer layer 55 is blanket formedover the wafer 100 in FIGS. 4A and 4B. The material of spacer layer 55may be selected to have a high etching selectivity with dielectric hardmask layer 30. For example, the material of spacer layer 55 may beselected from AlO, AlN, AlON, TaN, TiN, TiO, Si, SiO, SiN, and othermetals and metal alloys.

As also shown in FIG. 5B, spacer layer 55 is formed as a conformallayer, with the thickness T1 of its horizontal portions and thethickness T2 of its vertical portions close to each other, for example,with a difference between T1 and T2 smaller than about 20 percent ofthickness T1.

An anisotropic etching is then performed to remove the horizontalportions of spacer layer 55, while the vertical portions of spacer layer55 remain, and are referred to as spacers 58 hereinafter. The resultstructure is shown in FIGS. 6A and 6B, which include a top view and across-sectional view, respectively.

When spacer layer 55 (FIG. 5B) is formed, thickness T2 of spacer layer55 is also equal to or greater than a half of width W1 (FIG. 1A) ofopening 44. As a result, as shown in FIG. 5B, the sidewall (vertical)portions of spacer layer 55, which sidewall portions are on oppositesidewalls of opening 44, merge with each other to fill an entirety ofthe opening 44. As a result, the portion of dielectric hard mask 30underlying opening 44 is fully covered by spacers 58. In someembodiments, spacers 58 fill an entirety, or the lower part, of opening44. On the other hand, as shown in FIG. 6A, widths W4 of openings 54 aregreater than two times thickness T2 (FIG. 5B) of spacer layer 55, sothat each of openings 54 has a center portion 54′ remaining not filled(in the top view) by spacers 58. As shown in FIG. 6B, dielectric hardmask 30 is exposed through openings 54′.

In FIGS. 7A, 7B, 8A and 8B, some undesirable mandrel portions, such aswhat are marked in dashed rectangles 57 in FIG. 6A, are etched.Referring to FIGS. 7A and 7B, which include a top view and across-sectional view, respectively, bottom layer 68, middle layer 70,and upper layer 72 are formed. The material of bottom layer 68, middlelayer 70, and upper layer 72 may be selected from the same candidatematerials of bottom layer 38, middle layer 40, and upper layer 42 (FIG.1B), respectively. Upper layer 72 is patterned to form openings 74(including 74A, 74B, and 74C) therein. Mandrel 56A is overlapped byupper layer 72. Mandrel 56B is overlapped by opening 74B, while a firstportion of mandrel 56C is overlapped by opening 74C, and a secondportion is overlapped by upper layer 72.

Next, as shown in FIGS. 8A and 8B, which include a top view and across-sectional view, respectively, an etching process is performed toremove portions of mandrels 56B and 56C (FIGS. 6A and 6B). A firstportion of mandrel 56D is removed, while a second portion of mandrel 56Dremains. As shown in FIGS. 8A and 8B, openings 83 (including 83A and83B) are formed between neighboring spacers 58. Spacers 58 include edges58A and 58B. Edges 58A are formed due to the etching of spacer layer 55(FIGS. 5A, 5B, 6A and 6B). Accordingly, edges 58A are slopped. Edges 58Bof spacers 58 are formed due to the removal of mandrels 56. Sincemandrels 56 have top widths greater than the respective bottom widths,the resulting edges 58B are also slopped. Accordingly, openings, forexample, opening 83B, have top width greater than the respective bottomwidths. Openings 83 are thus referred to as having horn shapes. Inaddition, openings 54′ also have horn shapes. This is beneficial for thesubsequent gap-filling process as shown in FIG. 10. As a comparison, ifmandrels 56 have vertical edges, opening 83 will have vertical edges,and the subsequent gap-filling is more difficult.

Referring to FIG. 9, mandrels 56 and spacers 58 are in combination usedas an etching mask to etch the underlying dielectric hard mask 30 andlow-k dielectric layer 28, so that trenches 84 are formed. Additionalprocess steps are also performed to define and etch low-k dielectriclayer 28 and dielectric layer 27 to form via openings 86 underlyingtrenches 84. Etch stop layer 26 is also etched. Conductive features 126are exposed through trenches 84 and via openings 86. Although trenches84 and via openings 86 are shown as having the same widths in theillustrated plane, in a vertical plane perpendicular to the illustratedplane, via openings 86 have smaller widths than trenches 84.

As shown in FIG. 9, there is no metal hard mask layer (and there is asingle dielectric hard mask layer) between mandrels 56/spacers 58 andthe underlying low-k dielectric layer 28. Therefore, the patternsdefined by mandrels 56 and spacers 58 may be transferred directly intolow-k dielectric layer 28 without the need of transferring to a metalhard mask first and then transferred from the metal hard mask to low-kdielectric layer 28. The process steps related to otherwise neededetching and the rounding of the metal hard mask is thus no longerneeded. The manufacturing cost is saved. Furthermore, with less etchingeffort, the damage to low-k dielectric layer 28 is less severe.

FIGS. 10 and 11 illustrate the filling of conductive material 85 intotrenches 84 and via openings 86, and the removal of excess conductivematerial 85 (FIG. 10) to form metal lines 88 and vias 90 (FIG. 11),respectively. The formation may use a dual damascene process, wherein aconductive barrier layer such as titanium nitride, titanium, tantalumnitride, tantalum, or the like is formed on the sidewalls and thebottoms of trenches 84 and via openings 86. The remaining portions oftrenches 84 and via openings 86 are then filled with conductive material85, which may include copper or copper alloy. As shown in FIGS. 7A and7B, with openings 83 (FIG. 9) having top widths greater than therespective bottom widths, the entrances of openings 83 are larger thanvertical openings, hence the gap filling of conductive material 85 iseasier, with the likelihood of generating voids reduced.

A Chemical Mechanical Polish (CMP) is then performed to remove excessportions of the barrier layer and the filling metal, forming metal lines88 and vias 90 as shown in FIG. 11. Mandrels 56 and spacers 58 are alsoremoved by the CMP. Metal lines 88 and vias 90 are electricallyconnected to the underlying conductive features 126. The CMP may bestopped on low-k dielectric layer 28, as shown in FIG. 11, or may bestopped on dielectric hard mask layer 30.

In alternative embodiments, target layer 28 is a semiconductor material.Accordingly, the process step shown in FIGS. 1 through 10 may be used toform trenches in target layer 28, and filling the trenches with adielectric material to form Shallow Trench Isolation (STI) regions.

FIG. 12A illustrates a top view of metal lines 88 formed in low-kdielectric layer 28. As shown in FIG. 12A, metal lines 88 include 88A,88B, 88C, and 88D. Metal lines 88A and 88B are parallel to each other,and are closely located. Metal lines 88A and 88B are formed fromopenings 54′ (FIGS. 8A and 8B). Metal lines 88C and 88D are locatedbetween metal lines 88A and 88B. Metal lines 88C and 88D are formed fromopenings 83A (FIGS. 8A and 8B). The lengthwise directions (and thelengthwise center lines) of metal lines 88C and 88D are aligned to thesame straight line 21. In accordance with some embodiments, line endspace 51 between metal lines 88C and 88D is between about 5 nm and about100 nm. It is appreciated, however, that the values recited throughoutthe description are merely examples, and may be changed to differentvalues. As shown in FIG. 12A, metal line 88A includes main portion 88A1,which is rectangular, and tip 88A2 protruding beyond edge 88A3 andtoward the space between metal lines 88C and 88D. Similarly, metal line88B includes main portion 88B 1, which is rectangular, and tip 88B2protruding beyond edge 88B3 and toward the space between metal lines 88Cand 88D. The tip portions are formed due to the formation of spacers 58(including 58A and 58B), as shown in FIGS. 8A and 8B, wherein openings54′ have tip portions.

FIGS. 12B and 12C are cross-sectional views of the structure shown inFIG. 12A, wherein the cross-sectional views are obtained from thevertical plane containing lines A-A and B-B, respectively, in FIG. 12A.

The embodiments of the present disclosure have some advantageousfeatures. By forming mandrels having top widths greater than therespective bottom widths, it is easy to fill a conductive material suchas copper into trench openings and via openings. The likelihood ofcausing incomplete gap filling is thus reduced. With no metal hard maskformed under mandrels, the damage to the low-k dielectric layer causedby the metal hard mask patterning is avoided.

In accordance with some embodiments, a method includes forming a mandrellayer over a target layer, and etching the mandrel layer to formmandrels. The mandrels have top widths greater than respective bottomwidths, and the mandrels define a first opening in the mandrel layer.The first opening has an I-shape and includes two parallel portions anda connecting portion interconnecting the two parallel portions. Spacersare formed on sidewalls of the first opening. The spacers fill theconnecting portion, wherein a center portion of each of the two parallelportions is unfilled by the spacers. Portions of the first opening thatare unfilled by the spacers are extended into the target layer.

In accordance with other embodiments, a method includes forming amandrel layer over a target layer, performing a first etching step onthe mandrel layer to form a first opening in the mandrel layer, andperforming a second etching step on the mandrel layer to form a secondopening and a third opening parallel to each other, wherein oppositeends of the first opening are connected to the second opening and thethird opening to form an I-shaped opening. The method further includesforming a blanket spacer layer over the mandrel layer, wherein theblanket spacer layer extends into the I-shaped opening, and removingportions of the blanket spacer layer over the mandrel layer, withremaining portions of the blanket spacer layer forming spacers. Thespacers include a connecting portion filling the first opening, whereincenter portions of the second opening and the third opening are unfilledby the spacers. The mandrel layer is etched to remove portions of themandrel layer to form a fourth opening and a fifth opening in themandrel layer, wherein the fourth opening and the fifth opening arebetween the second opening and the third opening. The method furtherincludes using the mandrel layer and the spacers as an etching mask toetch the target layer to form trenches in the target layer. The trenchesare filled with a material, wherein the material includes portions overand contacting remaining portions of the mandrel layer and the spacers.

In accordance with yet other embodiments, a method includes forming amandrel layer over a low-k dielectric layer, performing a first etchingstep on the mandrel layer to form a first opening in the mandrel layer,and performing a second etching step on the mandrel layer to form asecond opening and a third opening parallel to each other, whereinremaining portions of mandrel layer comprise mandrels having top widthsand bottom widths smaller than the respective top widths. Opposite endsof the first opening are connected to the second opening and the thirdopening to form an I-shaped opening. The method further includes forminga blanket spacer layer over the mandrel layer, wherein the blanketspacer layer extends into the I-shaped opening, removing portions of theblanket spacer layer over the mandrel layer, with remaining portions ofthe blanket spacer layer forming spacers. The spacers include aconnecting portion filling an entirety of a portion of the first openingnot overlapping the second opening and the third opening, wherein centerportions of the second opening and the third opening are unfilled by thespacers. The portions of the mandrels are etched to form a fourthopening and a fifth opening in the mandrel layer, wherein the fourthopening and the fifth opening are on opposite sides of the connectingportion of the spacers, and are between the second opening and the thirdopening.

Although the embodiments and their advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the embodiments as defined by the appended claims. Moreover,the scope of the present application is not intended to be limited tothe particular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed, that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the disclosure.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps. In addition, each claim constitutes a separateembodiment, and the combination of various claims and embodiments arewithin the scope of the disclosure.

What is claimed is:
 1. A method comprising: forming a mandrel encirclingan I-shaped space, the mandrel comprising: a ring portion having arectangular top view shape, wherein the ring portion comprises a firstside portion and a second side portion; a first extension portionextending from the first side portion toward the second side portion;and a second extension portion extending from the second side portiontoward the first side portion, wherein the first extension portion andthe second extension portion have lengthwise directions substantiallyaligned to a same straight line, and the first extension portion and thesecond extension portion are spaced apart from each other by a portionof the I-shaped space; forming a blanket spacer layer over the mandrel,wherein the blanket spacer layer extends into the I-shaped space;etching horizontal portions of the blanket spacer layer, with remainingportions of the blanket spacer layer forming spacers; etching the firstextension portion and the second extension portion; and etching a targetlayer underlying the mandrel using remaining portions of the mandrel andthe spacers as an etching mask.
 2. The method of claim 1, wherein afterthe horizontal portions of the blanket spacer layer are etched, theI-shaped space has two portions remaining, and the two portions are onopposite sides of both the first extension portion and the secondextension portion, and are disconnected from each other.
 3. The methodof claim 1, wherein after the horizontal portions of the blanket spacerlayer are etched, the region between the first extension portion and thesecond extension portion is fully filled by a portion of the spacers. 4.The method of claim 1, wherein the etching the target layer results inopenings to be formed in the target layer, and the method furthercomprises filling the openings in the target layer with a conductivematerial.
 5. The method of claim 4, wherein after the filling theopenings in the target layer, the conductive material comprises aportion over and contacting the mandrel and the spacers.
 6. The methodof claim 5 further comprising performing a planarization to remove themandrel and the spacers.
 7. The method of claim 1, wherein the formingthe mandrel comprises forming an amorphous silicon layer.
 8. The methodof claim 1 further comprising, before the mandrel is formed, forming adielectric hard mask layer over and contacting the target layer, whereinthe mandrel is over and contacting the dielectric hard mask layer.
 9. Amethod comprising: forming a mandrel layer over a target layer, whereinthe mandrel layer comprises a first opening and a second opening, and anend of the first opening is connected to a middle portion of the secondopening; forming a blanket spacer layer over the mandrel layer, whereinthe blanket spacer layer extends into the first opening and the secondopening; performing an anisotropic etching on the mandrel layer, whereinsome portions of the blanket spacer layer remain to form spacers, andthe spacers comprise a connecting portion filling an entirety of thefirst opening, and wherein a center portion of the second openingremains unfilled after the anisotropic etching; etching the mandrellayer to form a third opening and a fourth opening in the mandrel layer,wherein the third opening and the fourth opening are on opposite sidesof the connecting portion of the spacers; using the mandrel layer andthe spacers as an etching mask to etch the target layer to form trenchesin the target layer; and filling the trenches with a conductivematerial.
 10. The method of claim 9, wherein the first opening and thesecond opening are formed in different etching steps.
 11. The method ofclaim 9, wherein after the anisotropic etching, the center portion ofthe second opening is fully encircled by the spacers.
 12. The method ofclaim 9, wherein in a cross-sectional view of the mandrel layer,portions of the mandrel layer have top widths greater than respectivebottom widths.
 13. The method of claim 9 further comprising etching themandrel layer before the blanket spacer layer is formed, wherein themandrel layer is etched using a process gas selected from the groupconsisting essentially of CF₄, HBr, Cl₂, O₂, and combinations thereof.14. The method of claim 9, wherein the target layer comprises a low-kdielectric material, and the filling the trenches comprises fillingcopper, and wherein the method further comprises performing a ChemicalMechanical Polish (CMP) to remove remaining portions of the mandrellayer.
 15. The method of claim 9, wherein in the anisotropic etching,horizontal portions of the blanket spacer layer are removed, andvertical portions of the blanket spacer layer remain to form thespacers.
 16. A method comprising: forming a patterned mandrel layerhaving an I-shaped opening; forming a blanket spacer layer over thepatterned mandrel layer, wherein the blanket spacer layer extends intothe I-shaped opening; etching horizontal portions of the blanket spacerlayer, with remaining portions of the blanket spacer layer formingspacers, wherein remaining portions of the I-shaped opening comprises afirst opening and a second opening having first lengthwise directionsparallel to each other, and the first opening and the second opening areseparated from each other by the spacers and the patterned mandrellayer; and etching the patterned mandrel layer to form a third openingand a fourth opening, wherein the third opening and the fourth openinghave second lengthwise directions aligned to a straight line, and thethird opening and the fourth opening are both between the first openingand the second opening.
 17. The method of claim 16, wherein thepatterned mandrel layer comprise mandrels having top widths and bottomwidths smaller than the respective top widths.
 18. The method of claim16 further comprising: using the patterned mandrel layer and the spacersas an etching mask to etch a low-k dielectric layer under the patternedmandrel layer to form trenches in the low-k dielectric layer; andfilling the trenches with a conductive material.
 19. The method of claim18 further comprising performing a planarization to remove the patternedmandrel layer and the spacers.
 20. The method of claim 16 furthercomprising etching a blanket mandrel layer to form the patterned mandrellayer, wherein the blanket mandrel layer is etched using a process gasselected from the group consisting essentially of CF₄, HBr, Cl₂, O₂, andcombinations thereof.